Cryostat control

ABSTRACT

Flow control for a cryostat in which the refrigerant flow rate is controlled by adding a contaminant to the refrigerant.

United States Patent Markum 1 May 27, 1975 CRYOSTAT CONTROL 3,413,82112/1968 Villaume ct a1. 62/514 3,415,078 1 l L' [75] Inventor: ArvelDean Markum, San Juan 2/ 968 62/5l4 Capistrano, Calif. Assignee? Generaly f Corporatiml, Primary Examiner-William F. O'Dea Pomona Cahf'Assistant Examiner Rona1d C. Capossela [22 Filed: p 25 1974 Attorney,Agent, or FirmAlbert J. Miller; Edward B.

Johnson [21] Appl. No.1 464,078

[52] US. Cl. 62/474; 62/502; 62/514;

' 137/13; 165/40 57 AB [51] Int. Cl. F25b 43/00 [58] Field of Search 1.137/13; 165/40; 62/85,

62/114, 195, 474, 475, 502, 511, 514 Flow control for a cryostat 1nwh1ch the refngerant flow rate is controlled by adding a contaminant tothe [56] References Cited refngeram- UNITED STATES PATENTS 3,270,7569/1966 Dryden 137/13 1 Claim, 3 Drawing Figures 1 6 REFRIGERANT Z6 -1MlXlNG CHAMBER SUPPLY AVE. STEADY STATE TEMP.

PATENTEDi-im' 27 5 SUPPLY REFRIGERANT CHAMBER 1 CRYOSTAT CONTROLBACKGROUND OF THE INVENTION Joule-Thomson effect cooling devices,commonly referred to as cryostats, as well known in the art to producecryogenic temperature levels. The cryostats may be employed to maintainradiation sensing devices at the extremely low temperatures required.Examples of conventional Joule-Thomson effect cryostats maybe found inU.S. Pat. Nos. 2,991,633, 3,095,711, 3,353,371, 3,415,078 and 3,431,750.I

In order to achieve a rapid initial cool-down, large coolant orrefrigerant flows are required in conventional cryostats. Only afraction of this cool-down flow is, however, needed for steadystateoperation of the cryostat. Thus, a cryostat designed to meet theinitial cool-down flow requirements would be inherently inefficientduring steady state operation, while a more efficient steady state flowdesign would have an excessively long cool-down period.

' Since in many cryostat applications the coolant or refrigerant flow islimited by the available supply, techniques have been developed toprovide sufficient cooldown flow without providing excessive steadystate flow. While certain self-regulating flow control mechanisms havebeen developed for cryostats, these mechanisms, which have been eitherthermal-mechanical, electro-mechanical, or chemical in nature, have beenrather complicated, overly complex and often prone to operationaldifficulties. All rely upon external forces, thus consuming energy suchas electrical power and all include at least some moving parts. In somecases the basic cooling characteristics of the cryostat have beenaltered by the flow regulating mechanism.

SUMMARY OF THE INVENTION The invention is directed to a cryostat flowcontrol in which the refrigerant flow rate is controlled by the additionof a contaminant or foreign fluid to the refrigerant. After initialcool-down, the contaminant, having a higher solidification point thanthe refrigerant, will solidify in the cryostat and cause a partial orcomplete refrigerant flow stoppage. When the refrigerant flow is thusreduced or stopped, refrigeration slows or ceases with a resultant risein cryostat temperature which in turn then melts the solidifiedcontaminant. The refrigerant flow will then resume until the temperatureis again reduced to freeze up or solidify the refrigerant contaminant.

The alternate freeze-up and melting cycle achieves a greatly reducedaverage steady state refrigerant flow rate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration ofa cryostat utilizing the control of the present invention.

FIG. 2 is an enlarged section view of a portion of the heat exchangertube of the cryostat of FIG. 1.

FIG. 3 is a graphical representation of the operational cycle of acryostat having the flow control of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The cryostat control of thepresent invention is applicable to any type of cryostat (counterflow,regenerative, Joule-Thomson expansion, etc.). For purposes ofillustration, a Joule-Thomson expansion cryostat 10 having a coiledtubing heat exchanger 12 and liquid refrigerant reservoirv 14 isillustrated in FIG. 1. A high pressure refrigerant gas supply 16provides refrigerant to the heat exchanger 12 through a control valve18. The refrigerant cooled in the inlet side of the heat exchanger 12 isexpanded through an expansion valve 20,

I or alternately through a nozzle or orifice, and collected in theliquid refrigerant reservoir 14. The liquid refrigerant is thendischarged from the cryostat 10 through a refrigerant exhaust 22 afterpassing through the other side (outlet side) or heat exchanger 12.

Initially, the refrigerant gas is at the same temperature as itssurroundings. When admitted to the cryostat 10 it passes throughtheinlet side of the heat exchanger 12 and out from the heat exchanger 12through the expansion valve or nozzle 20. As the refrigerant expandsthrough the expansion valve 20, it drops in temperature because of theJoule-Thomson effect. This lower temperature refrigerant is then forcedthrough the outlet side of the heat exchanger 12 and thereby decreasesthe temperature of the incoming refrigerant. This incoming refrigerantthen expands through the expansion valve 20 and drops to an even, lowertemperature than the preceding increment of refrigerant. This processcontinues until such time that the refrigerant becomes liquefied at theexpansion nozzle 20. The system then remains stabilized at the boilingtemperature of the refrigerant.

In order to effect control of the cryostat 10 in accordance with thepresent invention, a gaseous contaminant or foreign fluid is'introducedinto the refrigerant from a contaminant supply 24. A mixing chamber 26may be provided to uniformly distribute or disperse the contaminantvapor throughout the refrigerant supplied to the cryostat 10.Alternately other methods of agitation, stirring, or heating may beutilized for this purpose.

As illustrated most clearly in FIG. 2, once cool-down has been achieved,the contaminant 30, having a solidification temperature higher than thatof the refrigerant, will precipitate out of solution from therefrigerant and freeze-up. This will reduce and eventually block theflow of refrigerant through the heat exchanger tube 28. As therefrigerant flow is reduced, refrigeration slows or ceases until thecryostat temperature rises and melts the solidified contaminant.Refrigerant flow then resumes and decreases the cryostat temperatureuntil the contaminant blockage occurs again. The cycle of alternatefreeze-up and melting occurs indefinitely until the refrigerant supplyis stopped. The operation of the cryostat is graphically illustrated inFIG. 3.

The type of contaminant, ratio of contaminant weight to refrigerantweight and the type of refrigerant can be varied to accommodate anydesired cooling cycle and cryostat configuration. The maximumtemperature reached during cycling, and the frequency of the cycling isdependent upon the percentage by weight of contaminant in therefrigerant gas supply.

In a 0.118 inch diameter, 1 inch long, finned tube cryostat, having agas-flow rate of 1.1 standard liters per minute of 16% Freon-l4 and 84%Freon-23 at a supply pressure of 500 pounds per square inch, 10 partsper million by weight of water vapor as a contaminant in the refrigerantwill cycle the refrigerated tip of the cryostat from 250 Kelvin toKelvin at about 10 second intervals. While the exact location of therefrigerant flow blockage was not determined, it is believed to occurnear or at the expansion nozzle.

Any desired coolant cycle can be tailored by proper selection of therefrigerant and contaminant in the proper proportions. A list ofpossible cooling cycles is provided below.

Temperature Range Refrigerant Contaminant l95- 275K Freon 23 Water Vapor145 275K Freon 14 Water Vapor 145 l65K Freon 14 Xenon l 12 165K MethaneXenon 88 120K Argon Krypton 78 120K Nitrogen Krypton 78 95K NitrogenMethane ever, the cryostat operating temperature is achieved, thecyclical freeze-up will significantly reduce the flow of refrigerantflow through the cryostat.

While specific embodiments of the invention have been illustrated anddescribed, it is to be understood that these embodiments are provided byway of example only and that the invention is not to be construed asbeing limited thereto, but only by the proper scope of the followingclaims.

What I claim is:

l. In combination:

a cryostat including a coiled tube heat exchanger;

a high pressure refrigerant gas supply to provide a refrigerant to saidcryostat, said refrigerant comprising a mixture of 16% Freon-l4 and 84%Freon-23; and

means to introduce a contaminant into the refrigerant for said cryostat,said contaminant comprising 10 parts per million by weight water vapor,said contaminant having a solidification point above that of therefrigerant to alternately freeze and melt in the coiled tube heatexchanger of said cryostat to reduce the flow of refrigerant throughsaid cryostat.

1. IN COMBINATION: A CRYOSTAT INCLUDING A COILED TUBE HEAT EXCHANGER, AHIGH PRESSURE REFRIGERANT GAS SUPPLY TO PROVIDE A REFRIGERANT TO SAIDCRYOSTAT, SAID REFRIGERANT COMPRISING A MIXTURE OF 16% FREON-14 AND 84%FREON-23, AND MEANS TO INTRODUCE A CONTAMINANT INTO THE REFRIGERANT FORSAID CRYOSTAT, SAID CONTAMINANT COMPRISING 10 PARTS PER MILLION BYWEIGHT WATER VAPOR, SAID CONTAMIANT HAVING A SOLIDIFICATION POINT ABOVETHAT OF THE REFREGERANT TO ALTERNATELY FREEZE AND MELT IN THE COILEDTUBE HEAT EXCHANGER OF SAID CRYOSTAT TO REDUCE THE FLOW OF REFRIGERANTTHROUGH SAID CRYOSTAT.